US cuts off Syria talks with Russia as Putin backs out of nuke deal

After days of threats, the U.S. officially cut off negotiations with Russia over the Syrian civil war on Monday, the cessation of talks coming only hours after Russia backed out of a joint deal with the U.S. to dispose of nuclear material.

In a statement released Monday afternoon, State Department spokesperson John Kirby accused Russia of "intensified attacks against civilian areas" and being responsible for a Sept. 19 attack on a humanitarian aid convoy.

"This is not a decision that was taken lightly," Kirby said in the statement. "The United States spared no effort in negotiating and attempting to implement an arrangement with Russia aimed at reducing violence, providing unhindered humanitarian access, and degrading terrorist organizations operating in Syria...Unfortunately, Russia failed to live up to its own commitments."

As part of the suspension, the U.S. is withdrawing personnel that it had dispatched to take part in the creation of a joint U.S.-Russia center. That center was to have coordinated military cooperation and intelligence if the ceasefire had taken hold. The suspension will not affect communications between the two countries aimed at de-conflicting counter-terrorism operations in Syria.

President Vladimir Putin earlier Monday suspended a Russia-U.S. deal on the disposal of weapons-grade plutonium. Putin's decree released by the Kremlin cited Washington's "unfriendly actions" and the United States' inability to fulfill its obligations under the 2000 deal as reasons for the move.

"The decision by the Russians to unilaterally withdraw from this commitment is disappointing," White House spokesman Josh Earnest said.

Under the agreement, which was expanded in 2006 and 2010, Russia and the U.S. each were to dispose of 34 metric tons of weapons-grade plutonium, enough material for about 17,000 nuclear warheads.

When it was signed, the deal was touted as an example of successful U.S.-Russian cooperation on nuclear non-proliferation.

Russia said last year it had started up a plant that produces mixed-oxide commercial nuclear reactor fuel known as MOX from weapons-grade plutonium. Meanwhile, the construction of a similar U.S. plant in South Carolina has been years behind schedule and billions of dollars over budget.

The U.S. administration wants to cancel the Savannah River Site's MOX project and use an alternative method for disposing of excess plutonium.

Putin pointed to the stalled plant earlier this year to accuse the U.S. of failing to meet its end of the deal. He also argued that the policy change would give the U.S. "return potential," or a chance to recycle the material back into the weapons-grade plutonium.

Commenting on Putin's move, the Russian Foreign Ministry said Monday that the U.S. has "done all it could to destroy the atmosphere encouraging cooperation." It cited U.S. sanctions against Russia over the Ukrainian crisis and the deployment of NATO forces near Russian borders as examples.

"We would like to bring Washington back to understanding that it can't introduce sanctions against us in areas where it's quite painless for the Americans, and at the same time continue selective cooperation in areas it sees as advantageous," it said.

A strain in U.S.-Russian ties escalated in recent weeks followed the collapse of a truce in Syria and the Syrian army's massive onslaught in Aleppo under the cover of Russian warplanes.

The Russian Foreign Ministry said Moscow would be ready to restore the plutonium agreement if the U.S. takes Russian concerns into account.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.